Beta ray light source structure



July 12, 1966 l. FEUER BETA RAY LIGHT SOURCE STRUCTURE Filed April 9,1963 Z l I r++vr&v+ .+..&++++v v+ I \R g UV M/ 7 I n 0 w m w m 0 5 a; J1 1 #4 4 4a I +91O H F 1% //J/ 2 7 INVENTOR. fem/g F505;? BY flu 94mm,wan aw ATTORNE Y phosphors are excited by a radioactive material.

United States Patent 3,260,846 BETA RAY LIGHT SOURCE STRUCTURE IrvingFeuer, Elmhurst, N.Y., assignor to Canrad Precision Industries, Inc, NewYork, N.Y., a corporation of New York Filed Apr. 9, 1963, Ser. No.271,770 7 Claims. (Cl. 250-77) The present invention deals with lightsources wherein More specifically, the invention relates to a systemwherein one or more regions or phosphor in combination with a heavymetal reflecting region serves to give a highly eflicient means forconverting radioactive energy into light.

It is well known that radioactive materials, i.e., beta ray emitters,can be employed to excite phosphors and cause the phosphor to give offlight. A wide variety of systems have been taught for accomplishing thisend. Thus, for example, flashlight or signs have been described whichconcentrate the light produced by the emitted radiation by the use of aparabolic mirror or straight forward aluminized reflector. The aluminumserves as a light reflector, thus tending to concentrate the usefullight given off during the process. However, all of these prior artsystems are relatively ineflicient, e.g., exhibit efliciencies of theorder of 1 to 4 percent (based on utilization of a minimal backscatterfactor).

In accordance with the present invention, means are taught for makingmaximum use of radioactive energy in terms of providing a light source.Moreover this is accomplished in a manner, insuring safety and utilizingsubstances for dual roles.

As employed in the specification, the terms forward or front are used todenote the areas of regions between the radioactive source and theexternal environment to be illustrated. Similarly, the term back regionis employed to denote the area behind the radioactive material and awayfrom the area or surface from which light is directed to the externalenvironment.

In accordance with the present invention, a light source which utilizesthe beta ray energy of a radioactive material, preferably a weaker'betaray transmitter (emits beta rays of less than 1 rnev. energy) ischaracterized by having a front phosphor region of sufficient depth toabsorb =beta rays given off by the source but not the light which isgenerated by the radioactive excitation of the phosphor particles, and aback heavy metal reflecting region which due to the high atomic numberof the metal, i.e., greater than, or equal to, 45 preferably greaterthan 76, which serves to back scatter the beta particles, as well asreflect light. The reflected beta particles then further excite theforward phosphor regions and ultimately this energy is discharged fromthe system in the form of light energy. When employing weak betaemitters giving off less than 0.2 rnev. beta rays a second phosphorregion is preferably positioned between the radioactive source and theback heavy metal reflecting region so as to further convert beta rayswhich are reflected by the heavy metal region and convert theradioactive energy into light energy which is sent forward- 1y out ofthe structure.

It is essential that the backscattering region be characterized as ametal or metal composite having an atomic number greater or equal to 45in order to reflect at least 60 percent of the back directed side rays.Materials of lower atomic number such as aluminum cannot serve toeffectively backscatter the beta particles. By use of the heavy metalregion beta particles which would normally be absorbed outside of thephosphor light producing material are now more efiiciently utilizedwithin the phosphor regions. Thus it is possible by the use of thepresent system to use merely the heavy metal back reflector region and afront phosphor region and get higher efliciencies in terms of lightconversion than would be effected by the use of a two layer phosphorsystem with or without the use of an aluminum reflecting surface, as forexample, shown in US. Patent 2,953,634 to MacHutchin et al.

The present invention is particularly more suited to the use of weakerbeta ray emitters, i.e., materials giving off beta rays having an energylevel of from 5 kev. to 1 mev. Examples of such beta ray emitters arecarbon 14, nickel-63, cesium-134, krypton-85, tritium H-3, sulfur- 35 orpromethium-l47. In one aspect of the present invention the radioactivematerial, e.g., tritium, may be deposited on an intermediate phosphorlayer. This is desirable because the radiation from the tritium ismaximally utilized and the upper non-activated phosphor layer serves asa protective barrier against radioactive contamination and mishandling.Through suitable modifications of the described structure, principallymodifications of thickness of the various regions, the structure can beutilized for converting more energetic beta emitters such as RaE andSr-90-Y9O into light energy.

In the high energetic emitter case it is preferable to have a highatomic reflector and only one phosphor lay- M (on the front) as there isminimum attenuation of the beta rays. There is however a higher lightattenuation in the two equal layers of phosphor. A convenient indicationof the energy levels of various beta emitters is set forth below.

Table 1 Weak Beta Emitters Medium Beta Emitters Strong Beta Emitters Name Energy, Name Mev. Name Mev.

Mev.

155 KR- 695 Y 2. l8 0(1}; Thallium-204. 765 RaE 1. 17 1 i 067 Numeroustypes of phosphors or phosphor combinations such as zinc sulfides,cadmium sulfides, zinc silicates, zinc beryllium silicates, zinc oxides,calcium tungstates, etc., are employed in the present structure. Thedepth of the front phosphor region will vary somewhat depending on theenergy level of the radioactive source but will be of sufficient depthso as to absorb beta rays but not light rays. This is made possible bythe fact that the attenuation thickness of optical transmission issubstantially greater than the beta ray thickness for completeabsorption of weak beta rays from the radioactive source, e.g., tritium.Thus the depth of the front phosphor region may be controlled to fallwithin a region giving at least 90 percent absorption of the weak betarays, without absorbing substantial quantities of the light ray. Thus,for example, when employing zinc sulfide or cadmium sulfide phosphors incombination with a tritium beta ray source, a thickness of 1 mil allows90 percent of the light rays to pass through unabsorbed while at least90 percent of the weak beta rays are absorbed, since the absorptionthickness for weak beta rays is of the order of l.020 microns (dependingupon the nature of the absorber).

The average particle size of the phosphor preferably lies in the micronrange, e.g., 2 to 30, especially 10 to 30 microns. This is desirablebecause there is little self attenuation of the light in thin layers.However, if particle size is too small there are large light scatterlosses.

As noted previously, it is essential that the back heavy metal region beof a metal or metal laminate having an atomic number greater than orequal to 45 in order to effectively return the beta rays to the forwardpart of the system and to effectively convert their energy into light.Simultaneously the heavy metal reflects light forwardly, thus giving ahighly effective overall conversion of radioactive energy to lightenergy. It is particularly preferred to employ platinum, osmium,iridium, and their alloys, as the heavy metal back reflecting region.Alternatively, bismuth or lead or high atomic weight oxides such as leadoxide can be employed. Additionally, a bound laminate of aluminumdeposited on a heavy metal such as platinum, bismuth, or lead can beutilized, the aluminum deposit serving to improve light reflection. Itshould be noted that the heavy metal region (compound of heavy metalwith or without bound aluminum) is positioned closest to the backphosphor region (if one be employed) and is separated from the frontphosphor region by the radioactive source. This is necessary since theback reflector region is employed to reflect both light and beta raysforwardly to the area where it is discharged from the structure in theform of light rays.

It is noted that since the front phosphor region is of suflicient depthso that at least some portions thereof are not radioactive, it serves asa protective cover absorbing the beta rays, as well as being a source oflight and thus no additional protective covers are necessary. Normally,however, it will be desirable to use a front glass or plastictransparent cover such as one made of methyl methacrylate or mica.However, no distinct radio-active absorbing protecting structure isrequired. It is desirable, however, to coat the internal surface of thetransparent cover with an anti-reflecting coating such as magnesiumfluoride so as to minimize the internal reflection of the emitted lightrays and thus maximize the effective light sent outwardly to theexternal environment. The various aspects and modifications of thepresent invention will be made more clearly apparent by reference to thefollowing description and accompanying drawings.

FIGURE 1 illustrates a system characterized by the use of a single frontphosphor region in combination with a heavy metal back reflectingregion.

FIGURE 2 illustrates the use of multiple phosphor regions in combinationwith a solid radioactive source.

FIGURE 3 depicts a system amendable to the utilization of 'agaseousradioactive material.

With reference to FIGURE 1, shown therein is a system characterized bythe use of a front phosphor region and solid radioactive materialsimbedded in a phosphor layer, there being no distinct back phosphorregion employed in the illustrated system. The entire system is enclosedin casing 1 which may be made of any of a wide variety of materials suchas glass, plastics, methacrylates, epoxies and metals, such as aluminumor iron. Casing 1 in combination with transparent glass or plastic cover5 provides an enclosure for containing the system of the presentinvention whereby beta rays are converted into light. The source ofradioactivity in region 7 are radioactive particles imbedded in or onphosphor grains. The actual impregnation of the phosphor particle Withthe radioactive solid can be done 'by a wide variety of conventionaltechniques, as for example, (a) sedimentation and evaporation, (b)vacuum evaporation, (c) slush milling and evaporation, (d) spraycoating, etc. The radioactive solid is a stearic type solid and a ZnSphosphor is employed. The radioactive material gives off beta rayshaving energy levels of approximately 3 kev. to 17.9 kev. The phosphorparticles are approximately 1 to 24 microns in size and the radioactivematerial comprises about 10*" to l percent (by weight) of the phosphor.Region 7 is approximately -18 microns in depth.

Positioned forwardly from said radioactive source is.

front phosphor region 8. Region 8 may contain 1 or more layers ofphosphor particles which are excited by the beta rays given off fromregion 7 and thus convert the radioactive energy into light energy whichpasses outwardly through transparent cover 5. At least a substantialportion of region 8 is free of radioactive materials so as to serve as ashield layer, preventing the weak beta rays from passing out throughtransparent cover 5. By the same measure the width of phosphor layer 8is such that the light produced therein is not absorbed to a substantialdegree and thus passes out to the external source. Phospor particles 4may be the same type of phosphor containing material employed in theradioactive region or alternatively can be a different phosphorcontaining material, as for example, in the present illustration,calcium tungstate. In general, there is no purpose for another phosphorin the coverage light source as another phosphor would yield anothercolor.

Numeral 6 in the drawing represents the radioactive substance depositedon or impregnated in phosphor particles 3.

'Since the beta rays are being given off in a variety of directions,normally only those passing forwardly would be seen by light producingphosphor region 4. However, in accordance with the present invention,region 2 containing a heavy metal, i.e., a platinum layer, is p0-sitioned behind the radioactive region 7 and serves to reflect both betarays and light which may be directed inwardly from phosphor regions 7and 8. The reflected light and backscattered beta rays are reflectedforwardly into phosphor region 8 and are effectively made use of, thelatter being converted to light energy upon impinging the phosphorparticles, and the former passing substantially unabsorbed out throughtransparent cover 5. In general, heavy metal reflecting region 2 willhave a thickness of approximately 0.1 mil to 10 mils, preferably 0.1 to2 mils, so as to effectively serve to reflect beta ray particles. Thus,in the present example, region 2 will have a; depth of about 0.5 mil;region 7, a depth of about 15 microns and region 8, a depth of about15-30 microns. Substantially no beta rays thus pass out of the systemthrough cover 5 while converting the beta rays of the radioactive solidsource material to light rays.

In general, it is desired that the various regions, e.g., phosphorregions, heavy metal reflecting regions be disposed in parallel relationin order to obtain uniformity of light discharged from the structure.-While parallel curved surfaces can be employed, in general it isdesirable to employ relatively flat regions.

Turning to FIGURE 2, shown therein is a particularly preferredembodiment of the present invention employing a plurality of phosphorvregions in combination with a heavy metal reflecting region. The sourceof beta rays are zinc sulfide particles having a tritiated center (about10* to l( weight percent tritium based in zinc sulfide). The centralradioactive solid source region is shown as a single layer of tritiatedzinc sulfide particles although a plurality of layers could, of course,be employed. Throughout the structure various binders, plasticizers,etc., can be employed to bind the various particles to each other or tosurfaces of the composite structure. Inorganic adhesives, such as sodiumsilicate and potassium silicate are particularly desirable because oftheir stability. Additionally, various resins such as epoxy resins orethylcellulose can be employed. The binders, plasticizers, etc., areindicated by the numeral 103 in the drawing.

A front phosphor particle region 109 is positioned between radioactivematerials 106 and the light discharging portion of the overallstructure. The present example phosphor region 109 contains one or morelayers of zinc sulfide phosphor particles 102. Particles 102 are 18microns, average, in size. The depth of region 109 is about 18 microns.

Positioned behind the radioactive source is a second phosphor region 108similarly containing zinc sulfide particles. Beta'rays given off by thetritium pass randomly and thus the presence of back phosphor layer 108serve to convert beta rays passed backwardly into light energy. Lightfrom regions 108 and 109, together with beta rays which are not emittedin a forward direction strike heavy metal reflecting region 101 which inthe present example is a platinum reflector having a thickness of 0.5mil. The heavy metal serves to reflect both the light and the betaparticles forwardly. The reflected beta particles then come into contactwith the phosphor in region 108 or 109 and are converted into lightenergy which passes out directly, or through reflection, through thefront surface of the light producing system. Instead of platinum, leadoxide, platinum-iridium alloy rhodium, etc., could be employed forregion 101.

A glass or a plastic, e.g., methyl 'methacrylate, cover 107 is normallyemployed at the front surface of the structure. Preferably the glass hasan internal anti-reflecting region 105 which may take the form ofmagnesium fluoride which has been previously deposited on the internalportions of the glass. The magnesium fluoride insures that emitted lightis not internally reflected into thecentral portions of the structure,but rather passes out through the glass covering plate. Enclosure 100surrounding the light source may be made of Lucite or any of a widevariety of conventional materials.

The relative dimensions of the system are as follows:

Approximate depth of front phosphor region 18 microns. Approximate depthof back phosphor region 18 microns. Approximate depth of heavy metalreflection region 0.5 mil. Overall depth of light source 1.5 cm. Overalllength of light source 5 .0 cm.25 cm. Overall width of light source 5.0cm.lO cm.

The tritium radioactive material has a radioactivity ranging from 2.5millicurie/cm. to a few hundred millicurie/cmf By operating inaccordance with the present invention, a light brightness level (havinga higher efiiciency as previously stated) ranging tflOm 5 microlambertsto a few hundred microlamberts is obtained. The efliciency of convertingthe beta rays into light energy can be better than 2 microlamberts permillicurie of solid tritiated compound in the low level light range.This is based on photometric measurements using an Amincophotomultiplier photometer and tritiated luminous standards.

FIGURE 3 illustrates a structure particularly suitable for use insystems wherein a gaseous radioactive material, such as krypton-85 ortritium (H-3) are employed. The system of FIGURE 3 is quite similar toFIGURE 2 in that it contains two phosphor particle regions, 202 and 203positioned on each side of radioactive region 206. Normally region 206is evacuated through port 207 and thereafter radioactive gas is injectedthrough inlet 207 to reach the pressure desired. Normally atmospheric orsomewhat less than atmospheric pressure is utilized. Light source 200similarly contains a heavy metal back reflecting layer 201 which servesto reflect both light and beta rays forwardly, light ultimately passingthrough transparent cover 205. The phosphor particles may be of any of awide variety, e.g., zinc sulfide, cadmium tungstate, etc. The thicknessof the front phosphor region in particular is chosen so as to absorbsubstantially all the beta rays emitted from region 206 in a forwarddirection While allowing the light generated by the excitement of thephosphor particles to pass outwardly. Structure 200 may be enclosed bywalls 204 which may be made of aluminum.

Cell 200 is gas tight and may possess a dehydrating agent such as silicagel therein. In the present example, the space between phosphor regions203 and 202, i.e., the depth of the radioactive region 206 is of theorder of 1 centimeter, and the phosphor regions have an approxi-' matedepth of about 18 microns. It is also to be noted that the overall depthof the cell, i.e., 1.5-3 centimeters is only a fraction of the otherdimensions of the cell, e.g., length, 25 cm.; width, 7.5 cm.; and thusmaximum efficiency may be approached from the geometrical and reflectiveproperties of the configuration.

It should be clearly understood that the present light sources can beemployed in a variety of manners. They can be employed for railway andsignaling purposes. They find application as a lantern or as a marker orsign; when employing it for the latter purpose a portion of the coveringplate may be made opaque and so the transparent portion is illuminatedand produces a self-luminous form such as a traflic speed indicator ordirectional signal, portable map reader or negative X-ray copier orreader.

Various modifications may be made to the present invention. For the moreenergetic medium energy beta emitter such as KR- (gaseous type) andThallium-204 (solid type) one may employ the basic combination of aheavy metal backscatterer and light reflector couple-d with a singlephosphor layer on the front face to produce a more effective lightsource. In this case the light attenuation produced by both a front andback phosphor can be appreciable; hence one would want maximumreflection of the beta rays.

With reference to the gas systems; one may utilize solely a single thickphosphor front screen especially for the more energetic emitters coupledwith a heavy reflective scatterer as in this case the total lightattenuation produced in a front and back phosphor system becomessignificant.

In general it will be desired \for the present invention to take theform of -a radioactive light source employing a weak beta ray source inwhich substantially planar regions of heavy metal reflector, phosphorparticles, and radioactive particles are utilized. The heavy metalregion serves both as anelectron and light reflector. A minimal numberof layers of a phosphorized material containing a radioactive source,i.e., tritiated phosphors can be employed with a front non-radioactivephosphor region serving as a source of light through excitement by betarays as well as substantially absorbing all forwardly directed beta raysand insuring safety of the overall device.

Having described the present invention, that which is sought to beprotected is set forth in the following claims.

I claim:

1. An improved radio-active light source which comprises a casing havingdisposed therein a radioactive region containing a material giving otfbeta rays, a phosphor region positioned in front of said radioactiveregion in the direction of light discharged from said source, saidphosphor region being of suflicient thickness to absorb a substantialportion of beta rays without substantial absorption of light rays, alight reflective and beta ray-reflective heavy metal reflecting regionpositioned behind and enclosing the back portion of said radio-activeregion, said heavy metal having an atomic number of at least 45 andhaving a thickness sufficient to reflect beta rays, said metal beingpositioned adjacent to said radioactive region and in direct contactwith the beta rays given off by said radioactive region and having afront facing light reflecting surface so that said reflecting regionserves to reflect both light and beta rays forwardly, said forwardlydirected beta rays exciting said phosphor region and being convertedinto light.

2. A radioactive light source structure comprising a casing havingdisposed therein a radioactive region containing beta emitters, aphosphor region positioned between said radioactive region and the areawherein light is discharged from said structure, said phosphor regionbeing of suflicient depth to absorb at least of the weak beta raysemitted from said radioactive region without substantially absorbinglight rays, a light reflective and beta ray reflective heavy metalreflecting region positioned behind and enclosing the portion of saidradioactive region away from the area of light discharged from saidstructure, said heavy metal region comprising a metal having an atomicnumber of at least 45 and being of a thickness suflicient to back-scatter a major portion of the beta rays contacting its structure,said metal portion being adjacent said radioactive region and in directcontact with the beta rays given off by said radioactive region andhaving a forward-facing light reflective surface so as to reflect bothlight and beta rays forwardly, said reflective beta rays and beta raysemanating from said radioactive region serving to excite the phosphorregion and be converted into light energy.

3. The structure of claim '1 wherein a transparent cover is positionedbetween said front phosphor region and the exterior area to beilluminated, said transparent cover being coated on its internal surfacewith a nonreflecting agent.

4. The structure of claim 1 wherein said heavy metal region comprisesplatinum.

5. A self luminous light source structure comprising in combinationtherein a phosphor region in the area wherein said light sourcestructure discharges light into the environment proximate thereto andfurther removed from the area where light is discharged from saidstructure a region which contains a substance emanating vweak beta rays,a second phosphor region positioned behind said region containing weakbeta ray source, a light reflective and beta ray reflective heavy metalreflective region positioned behind and enclosing the back portion ofsaid second phosphor region, said (heavy metal region being in directcontact with the beta rays emitted by said substance so as to be capableof reflecting light and beta rays forwardly, said heavy metal reflectingregion comprising a metal having an atomic number of at least 45, saidheavy metal reflecting region having a thickness sufiicient to reflectbeta rays and said reflective region having a front facing lightreflecting surface so that said reflecting region serves to reflect bothlight and beta rays for-wardly in the direction of said front phosphorregion, thus converting said beta rays to light energy, as well :asdirecting light to the front part of said structure, said phosphorregion being of a suflicient depth to absorb at least of the weak betarays without absorbing a substantial portion of the light emitted bysaid light source structure, light from said structure passing throughsaid front phosphor region and being directed outwardly from saidstructure therethrough.

6. The structure of claim 5 wherein said heavy metal region comprisesplatinum.

7. The structure of claim 5 wherein said weak beta ray source is tritiumimpregnated upon phosphor particles.

References Cited by the Examiner UNITED STATES PATENTS 2,721,274 10/1955Garbellano et a1. 25077 2,910,593 10/1959 Laing et .al. 250-77 2,953,6849/1960 MacH utchin et a1. 250-71 3,005,102 10/1961 MacHutchin et a1.25077 RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner.

1. AN IMPROVED RADIOACTIVE LIGHT SOURCE WHICH COMPRISES A CASING HAVINGDISPOSED THEREIN A RADIOACTIVE REGION CONTAINING A MATERIAL GIVING OFFBETA RAYS, A PHOSPHOR REGION POSITIONED IN FRONT OF SAID RADIOACTIVEREGION IN THE DIRECTION OF LIGHT DISCHARGED FROM SAID SOURCE, SAIDPHOSPHOR REGION BEING OF SUFFICIENT THICKNESS TO ABSORB A SUBSTANTIALPORTION OF BETA RAYS WITHOUT SUBSTANTIAL ABSORPTION OF LIGHT RAYS, ALIGHT REFLECTIVE AND BETA RAY-REFLECTIVE HEAVY METAL REFLECTION REGIONPOSITIONED BEHIND AND ENCLOSING THE BACK PORTION OF SAID RADIO-ACTIVEREGION, SAID HEAVY METAL HAVING AN ATOMIC NUMBER OF AT LEAST 45 ANDHAVING A THICKNESS SUFFICIENT TO REFLECT BETA RAYS, SAID METAL BEINGPOSITIONED ADJACENT TO SAID RADIOACTIVE REGION AND IN DIRECT CONTACTWITH THE BETA RAYS GIVEN OFF BY SAID RADIOACTIVE REGION AND HAVING AFRONT FACING LIGHT REFLECTING SURFACE SO THAT SAID REFLECTING REGIONSERVES TO REFLECT BOTH LIGHT AND BETA RAYS FORWARDLY, SAID FORWARDLYDIRECTED BETA RAYS EXCITING SAID PHOSPHOR REGION AND BEING CONVERTEDINTO LIGHT.